This project is oriented toward evaluating the mechanistic role for cellular membrane potentials as a part of the driving force for intestinal Na ion-dependent transport systems. Work for the current project period is aimed at employing fluorescent carbocyanine dyes in order to measure the magnitude of membrane potentials by non-invasive techniques, and at establishing the stoichiometry of Na ion: sugar entry in isolated intestinal epithelial cells. Erythrocyte ghosts and phospholipid vesicles will be employed as test systems for attempting to establish the relationship between dye fluorescence and membrane potential. These models are useful because they offer the possibility for imposing ion gradients of defined magnitude across a membrane boundary and for manipulation of the potential with the aid of specific ionophores. By employing impermeant anions, potentials can be calculated and correlated with the extent of dye uptake and total suspension fluorescence. When utilized in conjunction with a membrane system with Na ion-dependent transport capability the approach offers a way of defining the relationship between membrane potential and transport capability. The work related to establishing Na ion sugar coupling stoichiometry is of primary importance for determining the adequacy of transmembrane electrochemical potentials for Na ion as a driving force for observed steady-state gradients of sugar maintained by isolated enterocytes. The transport stoichiometry will be evaluated as a function of intracellular Na ion concentration and as a function of membrane potential in order to discriminate between various models which have been proposed for transport mechanism.